Study of 116,792 songs from 3,160 passerine species finds eight basic acoustic motifs
An international team led by Jean-Monnet University Saint-Étienne and CEFE Montpellier linked the choice of song motifs to species traits and sound propagation.
An international team led by researchers at Jean-Monnet University Saint-Étienne and the Centre of Functional and Evolutionary Ecology at the University of Montpellier analysed 116,792 songs from 3,160 passerine species worldwide. The results were published in Science science.org
Sciencescience.orgOpen source ↗. The recordings came from xeno-canto, a collaborative website created in the Netherlands in 2005, where anyone can share sounds of wild animals. The study was led by Nicolas Mathevon, a bioacoustics lecturer at Jean-Monnet University.
Eight motifs of the acoustic space
According to the authors, the acoustic space of passerine song is continuous but structured around eight poles, or elemental motifs. Three of them are whistles of different duration, three are trills of different speed, and two are more complex sounds: chaotic notes and harmonic stacks. Mathevon states that these eight motifs can describe any song. On average, a passerine species uses five of them, one of which dominates.
What shapes the choice of motif
The researchers combined the recordings with data accumulated over decades: beak morphology, social organization, competition for mates and habitat. They measured the weight of each factor statistically. The motif choice depends on a combination of biological traits (social organization, morphology, mating system) and the physics of sound propagation.
The more distant the individuals are from one another, the more they use simple motifs such as the flat whistle, which is less altered by distance. Beak size also matters: a large beak is harder to move quickly, and such species favour flat whistles, while smaller birds produce whistles with rapid variations. Monogamous species are less virtuosic than polygamous ones; with more rivals, birds produce more complex songs, with fast trills and modulated whistles.
Environment adds a further constraint. In tropical rainforests, humid air and dense vegetation hinder sound transmission, and flat whistles and slow trills are overrepresented there science.org
Sciencescience.orgOpen source ↗. Species in these forests live at low density, and the breeding season lasts almost all year, so sexual selection is comparatively weak.
In temperate regions the balance shifts. High population densities facilitate close-range communication, and short breeding seasons intensify sexual selection. Birds there favour complex, information-rich motifs such as ultrafast trills, despite their susceptibility to acoustic degradation. The authors conclude that the global geography of birdsong reflects a spatially varying equilibrium between physical constraints and the drive for complex communication.
